My kef 5.1 set comes with spikes on the sub, but I never truly understood why.

Whats the advantage/disadvantage of spikes vs rubber feet?

With a speaker, you want the enclosure to move as little as possible as it's shape will produce unusual harmonics and fire off in stupid directions if it vibrates.

Essentially you want the carefully designed driver to be doing all of the movement and creating all of the sound.

Spikes can allow speakers to be sat on floorboards through a carpet at best, or they can at least be much better anchored.

Due to the size of the driver and the amount of air it's shifting (you need to shift 4 times the amount of air to maintain the same volume for every octave lower your go in pitch (a halving of frequency)), only woofers and subs suffer from this problem.

The difference is noticable on my Mission floorstanders (which also have a chamber designed for filling with sand, again to prevent the speaker body moving)